JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. 9 SEPTEMBER 19, 1919 No. 15 



CHEMICAL CRYSTALLOGRAFUY.— Ammonium picrate and 

 potassium trithionate: optical dispersion and anomalous crys- 

 tal angles. HERBERT E. Merwin. Geophysical Laboratory. 



The optical properties of these two compounds were obtained 

 in connection with certain researches on other subjects, and are 

 placed on record here for the benefit of other microscopists. 



Ammonium Picrate. — The following observations were made on 

 four samples variously prepared : The crystals are orthorhombic ; 

 a sample recrystallized from water consisted of thin scales 1 1 

 (010) ; a sample not recrystallized contained slender prisms 

 elongated 1 1 c ; one recrystallized from ammonium hydrate con- 

 tained equant. grains. Measurements of two prisms corre- 

 sponded satisfactorily with previous measurements.^ Refrac- 

 tive indices observed microscopically were :a=i.5io,/3=i. 87© , 

 7=1 .9I0 . The orientation is o; || c, ^ \\ a, 7 1| b for light wave- 

 lengths greater than 541 ju/x; and a 1| c, /3 jj b, 7 || a for shorter 

 wave-lengths. 2E for546/x/i = 19°, 578)U)u = 51°, 6oofxfx = 60°, 

 635/^^ = 7i°> 675)UM = 32°, 528MM = 33°, and 513/iM = 45° 

 to 55°-^- 



The absorption from 700 /x/x to 500/i/x is a<0 = y. The 

 original crystals and those recrystallized from ammonium hy- 

 drate contained streaks which for 7 and |8 were much redder 

 than the main parts of the crystals. The streaks were optically 

 continuous with the rest of the crystal and showed no significant 

 differences in refraction. 



^ See discussion and observations by H. Baumhauer, Zeits. Kryst.49: 125. 191 1. 

 2 Baumhauer found for Li and Na values of 80° (about) and 56°. 



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